In carrying out chemical reactions and processes, it is important to know the surface area and pore volume of the catalytic materials employed, as these factors are related to the rate of reaction. As recognized in U.S. Pat. No. 2,788,657, issued to Innes Apr. 16, 1957, which is incorporated by reference herein, the catalytic reaction takes place on the surface of the catalytic material. Pore volume or structure is important, since it governs the diffusion of reactants and products to and from the surface of the catalytic material as well as exerting considerable influence upon the stability or life of the material.
One popular method for measuring the surface area and extent of porosity in powdered and granular solid materials involves the low temperature adsorption of an active gas from a mixture of an active and an inactive gas continuously flowing over a sample of the material. Nitrogen is most frequently the active gas and helium the inactive one. A typical mixture composition is 30% nitrogen and 70% helium. The use of nitorgen has been generally accepted as a standard method for determining surface area because of the close checks that could be obtained by such measurements where the area was known geometrically. Also, nitrogen is used because of its low cost, inertness and nonflammability.
In this method use is made of the phenomenon of adsorption of gas onto the surface of a solid at low (liquid nitrogen) temperatures. Gas sorption techniques utilize a theoretical model wherein the surface of the solid being measured is viewed as being covered by a monolayer of closely packed molecules of an adsorbed gas. If one can determine the amount of gas in the monolayer, the area covered by the monolayer can then be calculated from the product of the number of molecules in the monolayer multiplied by the cross sectional area of each molecule. See generally, U.S. Pat. No. 4,489,593 issued to Pieters, et al., Dec. 25, 1984, which is incorporated by reference herein.
In carrying out the conventional method the sample is initially heated to 150.degree. to 200.degree. C. with the mixed gas flowing about it to drive off gas and vapors picked up during exposure to the atmosphere and then cooled to room temperature and physically shifted to another position for analysis. The mixed gas flow is continued at the new location, but the sample temperature must be reduced to the temperature of liquid nitrogen to bring about adequate adsorption of nitrogen gas from the gas stream. This is generally accomplished by arranging the sample in a holder with inlet and outlet tubes extending upward so that a Dewar flask containing liquid nitrogen can be brought upward to immerse the sample and the lower portions of the tubes. Adsorption equilibration usually requires about 3 to 12 minutes for establishment.
Initially, the amount of N.sub.2 passing through the detector per unit of time is considered to be the baseline equilibrium level, arbitrarily set at zero. When the sample material is cooled to liquid nitrogen temperature, N.sub.2 is adsorbed onto the sample surface, causing a reduction in the amount of N.sub.2 detected downstream. This N.sub.2 drop continues until the adsorption of N.sub.2 forms a monolayer on the sample. Then the N.sub.2 level returns to the baseline equilibrium level, providing an indication that adsorption is complete.
When the sample is brought back up to room temperature, N.sub.2 gas is desorbed from the sample surface, causing an increase in N.sub.2 detected downstream. This increase continues until complete desorption has occurred. Then the N.sub.2 level returns to the baseline equilibrium level. The measurements are integrated and the surface area of the sample calculated from the amount of N.sub.2 adsorbed. During the measurement process, because of the physical conditions defined by the reaction equations, the pressure, volume and temperature of the N.sub.2 measured downstream from the sample should remain stable. This stability provides the basis for a useful comparison between calibration measurements and sample run measurements.
The adsorbed nitrogen is desorbed by warming the sample to room temperature by any of several known techniques. The simplest way is to wait for the sample to warm naturally by being in the ambient atmosphere, as suggested by U.S. Pat. No. 3,884,083 issued to Lowell May 20, 1975. Unfortunately, warming by this method is slow. Some analyzers require the sample to be brought to room temperature within about 30 seconds, otherwise the gas flow disturbance created by the temperature change will erroneously register as a gas quantity and distort results. U.S. Pat. No. 3,884,083 also suggests that, when using butane as an adsorbate, a heating mantle can be employed to warm the sample. The temperature of the sample is not easily regulated and overheating is quite possible.
U.S. Pat. No. 4,489,593 discloses an apparatus which has a temperature controlled box which performs several functions, including: (1) heating an incoming gas line; (2) compensating for the temperature sensitivity of the electronic circuitry; (3) ensuring that the temperature in the gas line is constant; and (4) eliminating the effect of changes in ambient temperature on the gas. Since the gas passes through the box before entering the sample holder, it appears that the heated gas would raise the sample temperature to above room temperature when coolant is removed. The thermistor measures the temperature of the air in the box, and not the temperature of the sample holder; control of the temperature of one component does not necessarily control the temperature accurately of both components.
Another method is to blow heated air over the sample chamber. This does not work well because of the lack of control of the temperature at the desired target of room temperature. Unless the temperature of the sample itself is monitored closely, the target temperature can be exceeded, which can cause inaccuracies in the data obtained. Also, the sample cell rapidly frosts over from atmospheric water vapor once removed from the coolant and this white frost reflects much of the thermal radiation, thereby showing the time of warming and making it difficult to know precisely when to stop warming the sample chamber.
U.S. Pat. No. 2,788,657, which is incorporated by reference herein, replaces the liquid nitrogen bath after cooling with a water bath at room temperature. This method requires the manual manipulation of a second container by the user. There is a need then, for a convenient, automated means for controllably bringing a cooled sample to room temperature, without exceeding room temperature. There currently does not exist a means which rapidly and controllably raises the sample temperature for desorption. Such a means should be integrated into an automated analyzer system and should not increase the amount of time or attention required by a user.